What happens if someone's stuff is in a car when it is repossessed?

Contact the repossession company. When the vehicle is received at the repossession storage lot, the contents are inventoried and secured. You may recover your personal items, but there may be a fee associated with this.

Just a heads up: do not attempt to pad your list of personal items, for instance: that original, long lost Picaso you were storing in the trunk; well, unless you can provide undeniable proof that you owned it, and it was absolutely in the trunk, you will not recover a dime for it when the repo company cannot produce it. And, more importantly, if you attempt this and it annoys the repo company sufficiently, they could bring attempted fraud charges against you. So, keep it real.